1. Field of the Invention
The present invention relates to an image sensing apparatus in which a series of unit cells each performing photoelectric conversion on light incident thereto are arranged on a semiconductor substrate in a one-dimensional array or in a two-dimensional matrix, in particular to a technology for improving the processing speed and light sensitivity of the image sensing apparatus.
2. Description of the Related Art
Late years, the development of a solid-state image sensing apparatus that comprises a solid-state image sensor having photoelectric converters and a drive unit thereof has been actively pursued.
The solid-state image sensing apparatus is largely used in digital cameras, such as digital still cameras and digital video cameras. There has been still a strong demand for improving image quality of digital cameras. Regarding solid-state image sensors, realizing higher-density pixels is now being accelerated at a rapid rate.
When a video is taken with a digital camera, it is not required to output the moving image at a very high resolution as for a still image. What is asked instead is to perform signal processing at a high speed.
For this purpose, a method has been proposed in which the number of pixels to be processed is reduced in the solid-state image sensors by a method of pixel skipping. In the solid-state image sensors, signal charges are generated through photoelectric conversion. According to the method, pixels are thrown out evenly throughout an image and the generated signal charges of only the remaining pixels are read out.
Additionally, Patent Document 1 (Japanese Laid-Open Patent Application No. H11-234688) for example discloses a solid-state image sensor drive method in which the number of pixels to be processed is reduced in the horizontal direction of the solid-state image sensor. According to the method, pixels are incorporated into blocks, each of which is made up of three pixels aligned next to each other in the horizontal direction. In each block, signal charges of two pixels except for the middle one (i.e. two at both ends of the block) are added together within the solid-state image sensor. The signal charge of the middle pixel is added to that of the middle pixel from the adjacent block. Thus, by adding signal charges of pixels, the disclosed drive method reduces the number of pixels to be processed in the horizontal direction of the video signals outputted from the solid-state image sensor.
In the case in which the number of pixels to be processed is reduced by one-third in the horizontal direction, a one-third component of the sampling frequency for outputting all pixels folds back and therefore the remaining two-thirds component is added to the DC component. In the solid-state image sensor driven by the above conventional method, however, the one-third component of the sampling frequency is not zero (see FIG. 15), which thereby leads to moiré and/or aliased signals, or reducing the light sensitivity due to a reduction in the sampling frequency. These incur deterioration in the image quality of the output video signals. In addition, adding signal charges of pixels causes an adverse effect in which the order of the video signals outputted from the solid-state image sensor becomes irregular, and therefore reordering of the data through image processing will be required.
In view of the above issues, the present invention aims to provide: (a) a solid-state image sensor which is capable of reducing the number of pixels to be processed, at least, in the horizontal direction of the solid-state image sensor and outputting high-quality video signals, free from moiré and aliased signals, at a high speed and with a high light sensitivity, as well as in the same orderly arrangement as when signal charges of all the pixels are read out, and (b) a solid-state image sensing apparatus and a camera provided with such a solid-state image sensor.    PATENT DOCUMENT 1: Japanese Laid-Open Patent Application No. H11-234688.